Evaluation of Possible Reproductively Mediated Character Displacement in the , rusticus and 0. sanbornii1

MARK J. BUTLER IV, Department of Biological Sciences, The Florida State University, Tallahassee, FL 323O6-3O5O

ABSTRACT. Orconectes rusticus is replacing several species of crayfishes in north-central and northeastern North America, including O. sanbornii in Ohio. Recent evidence suggests that the species replacements may be driven by asymmetrical reproductive success favoring 0. rusticus. Nonetheless, some sympatric associa- tions appear locally persistent. Because demonstrate size-assortative mating and there is a disparity in the sizes of the species, further divergence in the sizes of the species in sympatry could enhance reproduc- tive isolation, ultimately providing a mechanism for character displacement. To test this hypothesis the size differentials between crayfish collected from allopatric and sympatric populations in east-central Ohio were compared with expected differences. The possibility of clinal variation in size was addressed by comparing crayfish sizes along a continuous allopatric-sympatric-allopatric species gradient within one stream. The ini- tial character displacement hypothesis was not substantiated by comparisons of allopatric and sympatric populations within or among streams for male or female 0. rusticus or male 0. sanbornii. However, female 0. sanbornii size distributions were consistent with unilateral character displacement.

OHIO J. SCI. 88 (3): 87-91, 1988

INTRODUCTION that these deviations were consistent with character dis- The classic definition of character displacement placement. Orconectes rusticus was introduced into the (Brown and Wilson 1956) involves two closely related Licking River drainage (Knox and Licking counties, species with overlapping geographical distributions that Ohio) sometime after Turner's (1926) initial crayfish sur- are less similar in sympatry than allopatry. The original vey, but before 1967 (R. Jezerinac, unpubl. data). Thus explanation for character displacement centered on two the species have been interacting for at least 10 genera- mechanisms: interspecific competition or interspecific tions, a short but demonstrably sufficient time for the mating coupled with inviable hybrid offspring. Either evolution of a variety of traits in many groups process could favor the divergence of various key traits. (Doyle and Hunte 1981, Reznick 1982, Rice 1985, Grant (1972) later described character displacement in Endler 1986). Although 0. sanbornii had been extirpated more detail. Although he acknowledged the potential for from some stream sites by 0. rusticus since the last census reproductively based species divergence, he concentrated of the area (east-central Ohio; R. Jezerinac, unpubl. on competitive hypotheses. This perspective has per- data), many sympatric populations persisted. Similar situ- vaded the study of character displacement since then ations have been reported in southern Ohio (Flynn and (Slatkin 1980, Strong et al. 1984, Taper and Case 1985). Hobbs 1984), northern Wisconsin (Capelli 1982, Capelli Much of the recent criticism that research on character and Magnuson 1983, Lodge et. al. 1986), and Ontario displacement has received centers on the paucity of aut- (Berrill 1978, Tierney and Dunham 1984). ecological data available to substantiate claims of com- Rapid evolution of divergent traits requires strong petition and on the inability of many studies to reject the selection. Phenotypic correlations between species, par- alternative hypotheses of clinal variation or allopatric ticularly from only a few sites, cannot be accepted as character release (Grant 1972, Hespenheide 1973, Strong evidence for character displacement (Endler 1985). Thus et al. 1979, 1984, Grant and Abbott 1980, Strong and the existence of a sufficiently powerful selective agent Simberloff 1981). These objections are untenable in the must also be demonstrated. Several independent studies system described in this paper because 1) interspecific on crayfish mating and reproduction suggest that such a interactions driving species replacement and possibly mechanism may exist (Capelli and Capelli 1980, Capelli character displacement are documented and apparently 1982, Tierney and Dunham 1982, 1984, Berrill 1985, do not involve competition/WJ?; 2) allopatric-sympatric Butler and Stein 1985, Butler 1985, Lodge et al. 1985, population gradients are localized and are thus not sub- 1986). Orconectes rusticus males rarely engage in hetero- ject to geographical clines; and 3) sympatric associations specific copulations, whereas male 0. sanbornii and 0. are more recent than the allopatric condition, negating propinquus (a species closely related to 0. sanbornii and the possibility of character release. native to Wisconsin and Canada) frequently mate hetero- In a previous study I investigated the replacement of specifically with small 0. rusticus females (Tierney and a native Ohio crayfish, Orconectes sanbornii, by an intro- Dunham 1984, Butler and Stein 1985, Berrill 1985, but duced crayfish, Orconectes rusticus, and evaluated several see Capelli and Capelli 1980, Capelli 1982). Females that mechanisms that might drive species replacements (But- mate heterospecifically experience reductions in re- ler and Stein 1985). During that study we noted that the productive success of 50 to 90% (Berrill 1985, Butler and size ratios (i.e., carapace lengths and chelae lengths) of Stein 1985). Although the selection of small 0. rusticus the species were more disparate in sympatry (1.29 • 1; 0. females by 0. sanbornii (or 0. propinquus) males lowers rusticus vs. 0. sanbornii) than in allopatry (1.13: 1), and recruitment for all species, the relative recruitment of 0. rusticus is potentially greater. This probably occurs be- cause only large males (i.e., 0. rusticus) can copulate with 'Manuscript received 4 March 1987 and in revised form 31 August large 0. rusticus females (Tierney and Dunham 1984, 1987 (#87-13). M.J. BUTLER IV Vol. 88

Berrill and Arsenault 1982, 1984), and large females are 34 generally more fecund than small females (Fielder 1972, MALES Lorman 1980, Hazlett 1983, Salmon 1983). A mating 32- system of this type should precipitate a decline in the population size of the native species (0. sanbornii or 0. propinquus) unless the populations are subject to substan- tial immigration or the conflicting species become more reproductively isolated. Sympatric divergence in the relative size of the two species is one possible isolating mechanism. Most hetero- specific matings should occur between small 0. rusticus females and 0. sanbornii males because orconectid cray- fish mate with individuals of similar size (Berrill and Arsenault 1982, 1984), 0. rusticus is generally larger than 0. sanbornii (Turner 1926, Butler and Stein 1985), and FEMALES 0. rusticus males demonstrate greater species-specific mate selectivity (Tierney and Dunham 1984, Butler and Stein 1985). Individuals that mate heterospecifically will experience lowered reproductive success relative to other individuals in sympatric populations because their fe- CARAPAC E LENGT H (MM ) cundity is reduced and hybrids are apparently nonviable (Capelli and Capelli 1980, Smith 1981, Berrill 1985, Butler and Stein 1985). Thus, relatively larger 0. rusticus 24- and smaller 0. sanbornii might be selectively favored in sympatry, leading to character displacement in body size 22- and other highly correlated characters like chelae length 20 (Stein 1975, Stein et al. 1977). I tested this hypothesis 12 3456789 10 11 by examining the allopatric and sympatric size distribu- tions of two crayfish species within and among several STREAM SITE Ohio streams. FIGURE 1. Mean carapace lengths for male and female Orconectes METHODS rusticus (solid squares) and Orconectes sanbornii (open squares) at 11 sites along a 3-km transect of the North Fork Licking River, Licking and I visited a total of 32 stream sites in northern Licking and Knox Knox counties, Ohio. Note that the transect spanned two allopatric counties in 1984 and located only two 0. rustkus-0. sanbornii sym- zones and a zone of sympatry in between. Twenty-five to 35 crayfish patric populations and four allopatric 0. rusticus populations. The were collected at each site. remaining sites yielded only 0. sanbornii populations. Therefore, crayfish were collected from the two sympatric, the four allopatric 0. rusticus, and five nearby allopatric 0. sanbornii locales in five greater than 20 mm CL (i.e., those capable of reproduction; Lorman east-central Ohio streams in July, 1984. All five streams sampled are 1980, Fielder 1972, Berrill and Arsenault 1984) were used in the low gradient, third-order streams that drain agricultural watersheds. analyses. Orconectes rusticus was collected at four allopatric sites in the Otter Size-frequency data were first analyzed in one-factor, fixed-effects Fork, Muddy Fork, and North Fork of the Licking River and in analyses of variance (ANOVA; factor = collection site) to determine Raccoon Creek. Orconectes sanbornii was collected at five allopatric sites if sizes differed among sites. Bonferroni a priori multiple comparison in the North Fork and South Forks of the Licking River; both spe- tests (Kirk 1982) were then used to test whether crayfish size differed cies were taken at two sympatric locations in the North Fork of the between allopatric and sympatric sites. I also used Wilcoxon Rank- Licking River. Sum tests (Hollander and Wolfe 1973) to determine if ranks of the I examined interspecific size relationships on a smaller geographic mean sizes (CL) of both species and sexes at each site corresponded scale by comparing the sizes of crayfishes from 11 locations along a with the predicted size relationship (i.e., 0. rusticus should be larger 3-km transect of the North Fork of the Licking River. The transect in sympatry than in allopatry, whereas the reverse should be true for sampled spanned an upstream allopatric 0. sanbornii region, a down- 0 sanbornii). In another test, relative size differences between the two stream allopatric 0. rusticus region, and an area of sympatry in be- species in allopatry vs. sympatry were compared in two-sample t-tests. tween. Comparisons made between allopatric and sympatric sites Relative differences at each site, rather than absolute differences within a short section of stream alleviate the potential problem among sites, may be the most relevant parameter to measure with of differentiating character displacement from geographic clines in respect to natural selection. The dependent variable values for this body size. analysis were generated by computing the difference in the mean sizes (CL) of the two species in each possible pairwise comparison among Most adult 0. rusticus and 0. sanbornii reside in stream pools and runs (Butler and Stein 1985), so crayfish were collected only from sites within allopatry and sympatry. If character displacement had these habitats to reduce possible bias owing to size-specific habitat occurred, I expected a larger relative difference in size between spe- use. I used a handnet to collect crayfish and captured all crayfish cies in sympatry as compared to allopatry. Separate tests were per- sighted in the open or under rocks. At some sites, crayfish densities formed on males and females in all analyses. Identical analyses were were quite low and repeated sampling yielded only small sample sizes. performed on the data from different streams, as well as on the The sex, carapace length, and chela length of each crayfish were within-stream data. recorded in the field, and the were then released. Carapace RESULTS length (CL; anterior tip of rostrum to posterior mid-dorsal edge of the cephalothorax) served as the dependent variable in the analyses be- There were significant (ANOVA; P < 0.05) differ- cause it is a standard measure of crayfish size and is an accurate ences in the sizes (CL) of both 0. rusticus and 0. sanbornii indicator of aggressive dominance and mating success (Heckenlively males and females among stream locations. Crayfishes 1970, Rubenstein and Hazlett 1974, Berrill and Arsenault 1984, were generally smaller in sympatry than allopatry (Bon- Butler and Stein 1985). Carapace length is also significantly positively correlated with chela length (Stein 1975). Statistical analyses run on ferroni test; P < 0.05; Table 1). Orconectes sanbornii chelae lengths yielded results identical to those for carapace lengths, males and females were always smaller in sympatry (Wil- so only data for carapace lengths are reported. Only adult crayfishes coxon Rank-Sum test; P < 0.05), whereas the smallest Ohio J. Science CRAYFISH CHARACTER DISPLACEMENT 89

TABLE 1 Are these results compatible with other possible pre- Descriptive statistics for crayfishes collected in July, 1984 at 11 stream dictions of character displacement? Although both spe- sites in Licking and Knox counties, Ohio. Carapace lengths are in mm. cies experience lowered reproductive success in sympatry, The specific location of each site is given in Appendix Table 1. the effect on the native species is more acute (Butler and N = sample size. Stein 1985, Berrill 1985). Because selection operates on relative fitnesses (see Endler 1986 for review), a unilateral Carapace Length reduction in size by a native species, 0. sanbornii in this (* ± 1 SD) N case, could act as a prezygotic isolating mechanism pro- moting local sympatric persistence, a mechanism not Species Site Males Females Males Females previously considered. If this occurred, small 0. sanbornii Allopatry- females would be assured of only intraspecific copu- 0. sanbornii 1 26.5 (2.8) 27.3 (4.9) 12 9 lations, because the size-assortative mating system and 2 25.4 (4.6) 24.7 (4.8) 9 4 the larger size of 0. rusticus would thwart heterospecific 3 29.6 (5.4) 27.2 (3.6) 11 19 mating attempts by the nondiscriminantly mating 0. 4 26.4 (3.4) 25.7 (2.6) 37 62 sanbornii males. Confirmation of this hypothesis requires 5 26.9 (4.0) 26.9 (2.9) 19 16 0. rusticus 6 27.0 (3.6) 27.2 (4.6) 25 10 explicit comparisons of size-specific mating behavior and 7 33.1 (5.1) 28.6 (4.3) 30 21 reproductive success in allopatric and persistent sym- 8 26.8 (6.7) 24.3 (6.3) 7 4 patric crayfish populations. Those kinds of data are not 9 35.6(5.4) 35.3 (5.7) 5 3 currently available. Sympatry- Comparisons of the relative sizes of the species at spe- 0. sanbornii 10 24.4 (3.4) 23.8 (2.5) 27 26 11 25.4(2.9) 24.1 (2.9) 14 15 cific allopatric and sympatric sites may be the most valu- 0. rusticus 10 30.2(5.3) 27.2 (4.0) 52 26 able means of detecting character displacement, because 11 26.2 (6.1) 26.9 (4.8) 12 16 it is the relative size of individuals that drive species interactions. Thus, the customary approach of comparing 0. rusticus size ranks were evenly distributed among allo- absolute sizes among sites has little relevance to selection patric and sympatric locales (P = 0.27 and P = 0.40 and character displacement. In this study, 0. sanbornii for males and females, respectively). Relative differences was smaller than 0. rusticus in both sympatric streams in the mean (x) sizes of the species were similar in allo- and at four of five sympatric locations along the North patry and sympatry (t-test; P > 0.05), but 0. rusticus Fork Licking River transect. Despite the smaller size of males were_ generally larger than 0. sanbornii males in 0. sanbornii in sympatry, there was no statistical evidence allopatry (x = 3.7 mm difference) and sympatry (x = for larger interspecific size ratios in sympatry than in al- 3.3 mm). Similarly, female 0. rusticus were generally, lopatry. Whether the size differential between the species though not significantly, larger than 0. sanbornii fe- at sympatric locations is sufficient to preclude replace- males in allopatry (x = 2.5 mm difference) and sym- ment of 0. sanbornii is unknown and warrants further patry (x = 3.1 mm difference). investigation. Both species and sexes differed in size among sites The persistence of sympatric crayfish populations may along the North Fork Licking River transect (ANOVA; also be enhanced by a number of mechanisms besides P < 0.05; Fig. 1). Changes in the size of 0. rusticus character displacement. For example, high rates of immi- males did not correspond with interspecific association, gration, particularly movement from allopatry to sym- but 0. rusticus females and 0. sanbornii males and females patry, might swamp the effects of selection and extend were significantly (Bonferroni test; P < 0.05) smaller in local persistence. In fact, site-to-site variation in immi- sympatry. However, if sites were ranked by crayfish size gration due to differences in physical barriers or popu- for both species and sexes, only 0. sanbornii females were lation density is an appealing explanation for the variable consistently smaller in sympatry than allopatry (Wil- rates of replacement observed in seemingly similar lakes coxon Rank-Sum test: P < 0.005). The relative size differ- and streams (Rhoades 1962, Berrill 1978, Capelli 1982, ences between the two species in allopatry and sympatry, Butler and Stein 1985, Flynn and Hobbs 1984, Lodge by sex, were not significant (t-test; P > 0.05). On aver- et al. 1986). Segregation of species by habitat or differen- age, males differed by 4.8 mm CL in allopatry and tial susceptibility to predation are also plausible mecha- 6.2 mm CL in sympatry; females differed by 3.7 mm CL nisms that might sustain sympatric associations. There is and 3.2 mm CL in allopatry and sympatry, respectively. circumstantial evidence supporting these hypotheses (Penn and Fitzpatrick 1963, Schwartz et. al. 1963, DISCUSSION Bovbjerg 1970, Rorer and Capelli 1978, Medvick 1979, The results of this field survey provide no compelling Collins et al. 1983, Butler and Stein 1985). Behavioral evidence for bilateral (i.e., both species) displacement of reproductive isolation via chemosensory-controlled mate carapace length, either among or within streams, in sym- selection may explain why some sympatric Canadian patric orconectid crayfish populations. When data from crayfish assemblages appear stable (Ameyaw-Akumfi and individual collection sites within or among streams were Hazlett 1975, Itagaki and Thorp 1981, Tierney and pooled in an overall test for size disparities in allopatry Dunham 1982, 1984), although this hypothesis has not vs. sympatry, both species and sexes were smaller in been addressed explicitly. sympatry. However, the 0. rusticus data were strongly Finally, it might be argued that the size differences influenced by large deviations at just a few sites. When observed in sympatry may be due to differences in the the sites were considered individually, only 0. sanbornii two species' growth rates in sympatry, not character dis- was consistently smaller in sympatry (i.e., unilateral size placement. For example, 0. rusticus might monopolize divergence). food resources and inhibit the growth of 0. sanbornii. 90 M.J. BUTLER IV Vol. 88

This alternative seems rather weak, given that there is LITERATURE CITED no evidence that these species differ in aggressive ten- dencies or compete for food (Butler and Stein 1985). Ameyaw-Akumfi, C. and B. A. Hazlett 1975 Sex recognition in There is limited laboratory and field data available on the crayfish Procambarus clarkii. Science 190: 1225-1226. growth rates for the two species, but interspecific com- Berrill, M. 1978 Distribution and ecology of crayfish in the Ka- wartha Lakes region of southern Ontario. Can. J. Zool. 56: parisons are confounded by differences in laboratory 166-177. conditions and field sites (Fielder 1972, Lorman 1980, and M. Arsenault 1982 Spring breeding of a northern Butler 1983). Although there seems to be little support temperate crayfish, Orconectes rusticus. Can. J. Zool. 60: for this alternative, it should be noted that if this hy- 2641-2645. and 1984 The breeding behavior of a northern pothesis were true the effects on species persistence temperate Orconectid crayfish, Orconectes rusticus. Anim. Behav. 32: would be the same as character displacement, namely 333-339. greater divergence in the relative sizes of the species in 1985 Laboratory induced hybridization of two crayfish sympatry, reduced interspecific copulations, and en- species, Orconectes rusticus and 0. propinquus. J. Crust. Biol. 5: hanced reproductive isolation. 347-349. Bovbjerg, R. V. 1970 Ecological isolation and competitive exclu- In summary, the results of this study are consistent sion in two crayfish {Orconectes virilis and Orconectes immunis). with the hypothesis that 0. sanbornii may avoid replace- Ecology 51: 225-236. ment by 0. rust km via a unilateral reduction in size in Brown, W. L. and E. O. Wilson 1956 Character displacement. sympatry. Selection for the most fertile 0. sanbornii indi- Syst. Zool. 5: 49-64. Butler, M.J. IV 1983 An analysis of replacement mechanisms viduals, presumably the smaller size classes that are rela- governing range expansion in crayfish. Columbus: The Ohio State tively immune to the mating and reproductive anomalies Univ. Thesis. experienced by larger individuals, may drive the evo- and R. A. Stein 1985 An analysis of the mechanisms lution of smaller individuals. Several alternative hypothe- governing species replacements in crayfish. Oecologia 66: 169-177'. ses are tenable as well, but data that might discriminate 1985 Crayfish: tyrant or prude? Nat. Hist. 95: 4. among these competing ideas are either circumstantial or Capelli, G. M. and J. F. Capelli 1980 Hybridization between crayfish of the genus Orconectes: morphological evidence (: nonexistent. Further research on this topic should focus ). Crustaceana 39: 121-132. on size-specific mate selection and reproductive success, 1982 Displacement of northern Wisconsin crayfish by comparisons of allopatric and sympatric growth rates, Orconectes rusticus (Girard). Limnol. Oceanogr. 27: 741-745. interdemic rates of immigration, and chemosensory- and J.J. Magnuson 1983 Morphoedaphic and bio- geographic analysis of crayfish distribution in northern Wisconsin. mediated mate choice as possible mechanisms facilitating J. Crust. Biol. 3: 548-564. species coexistence in orconectid crayfish. and B.L. Munjal 1983 Aggressive interactions and re- ACKNOWLEDGMENTS. I thank K. Butler for his assistance in the source competition in relation to species displacement among cray- field and P. Greenwood, H. H. Hobbs III, D. Lodge, D. Wilber, and fish of the genus Orconectes. J. Crust. Biol. 2: 486-492. two anonymous reviewers for critiquing earlier drafts of this manu- Collins, N.C., H.H. Harvey, A.J. Tierney, and D. W. script. This research was conducted with the aid of a Sigma Xi Dunham 1983 Influence of predatory fish density on trapability Grant-in-Aid of Research and a Florida State University College of of crayfish in Ontario lakes. Can. J. Fish. Aquat. Sci. 40: Arts and Sciences Fellowship. 1820-1828. Doyle, R. W. and W. Hunte 1981 Demography of an estuarine amphipod {Gammarus lawrencianus) experimentally selected for high "r": a model of the genetic effects of environmental change. Can. APPENDIX TABLE 1 J. Fish. Aquat. Sci. 38: 1120-1127. Crayfish collection site locations in Licking and Knox counties, Ohio. Endler, J. A. 1986 Natural selection in the wild. Princeton, NJ: SR = state route; TR = township route. Princeton Univ. Press. Fielder, D. D. 1972 Some aspects of the life histories of three closely related crayfish species Orconectes obscurus, 0. sanbornii, and Site Location description 0. propinquus. OhioJ. Sci. 72: 129-145. Flynn, M. F. and H. H. Hobbs III 1984 Parapatric crayfishes in 1 Licking County, Burlington Township. North Fork southern Ohio: evidence of competitive exclusion? J. Crust. Biol. Licking River, 0.5 km east of the intersection of SR 661 4: 382-389. and TR 19. Grant, P. R. 1972 Convergent and divergent character displace- 2 Licking County, Harrison-Lima Township. South Fork ment. Biol. J. Linn. Soc. 4: 36-69. Licking River at TR 34. and I. Abbott 1980 Interspecific competition, island bio- 3 Licking County, Lima Township. South Fork Licking River at geography, and null hypotheses. Evolution 34: 332-341. TR 155. Hazlett, B. A. 1983 Parental behavior in decapod Crustacea. In: 4 Knox County, Hilliar Township. North Fork Licking River, S. Rebach and D. W. Dunham (eds.), Studies in adaptation: the 0.5 km west of TR 112. behavior of higher Crustacea. New York: J. Wiley and Sons; 5 Licking County, Bennington Township. North Fork Licking pp. 171-193. River at SR 657. Heckinlively, D. B. 1970 Intensity of aggression in the crayfish 6 Licking County, Bennington Township. Otter Fork Licking Orconectes virilis (Hagen). Nature 225: 180-181. River at TR 56. Hespenheide, H. A. 1973 Ecological inferences from mor- 7 Licking County, Bennington Township. North Fork Licking phological data. Ann. Rev. Ecol. Syst. 4: 213-229. River, 0.5 km east of TR 56 at TR 19. Hollander, M. and D. A. Wolfe 1973 Nonparametric statistical 8 Licking County, Newark Township, City of Newark. methods. New York: J. Wiley and Sons. Licking River, 0.25 km south of the intersection of SR 16 Itagaki, H. and J. H. Thorp 1981 Laboratory experiments to de- and SR 79- termine if crayfish can communicate chemically in a flow-through 9 Licking County, Lima Township. Muddy Fork Licking River system. J. Chem. Ecol. 7: 115-126. at TR 38. Kirk, R. E. 1982 Experimental design: procedures for the be- 10 Knox County, Hilliar Township. North Fork Licking River at havioral sciences. 2nd ed. Monterey, CA: Brooks/Cole Pub. Co. TR 19. Lodge, D. M., A. L. Beckel, and J.J. Magnuson 1985 Lake bot- 11 Licking County, Bennington Township. North Fork Licking tom tyrant. Nat. Hist. 94: 32-37. River, 0.25 km southeast of the intersection of TR 114 and , T. K. Kratz, and G. M. Capelli 1986 Long-term dynam- TR 19- ics of three crayfish species in Trout Lake, Wisconsin. Can. J. Fish. Aquat. Sci. 43: 993-998. Ohio J. Science CRAYFISH CHARACTER DISPLACEMENT 91

Lorman, G. 1980 Ecology of the crayfish Orconectes rusticus in north- Smith, D. G. 1981 Evidence for hybridization between two cray- ern Wisconsin. Madison: Univ. of Wisconsin. Dissertation. fish species (Decapoda: Cambaridae: Orconectes) with a comment Medvick, P. 1979 Aggressive interactions and predation avoid- on the phenomenon in Cambarid crayfish. Amer. Midi. Nat. 120: ance in young-of-the-year of three species of crayfish {Orconectes). 405-407. Madison: Univ. of Wisconsin. Dissertation. Stein, R. A. 1975 Sexual dimorphism in crayfish chelae: func- Penn, G. H. and J.F. Fitzpatrick, Jr. 1963 Interspecific com- tional significance limited to reproductive activities. Can. J. Zool. petition between two sympatric species of dwarf crayfishes. Ecology 54: 220-227. 44: 793-797. , M.J. Murphy, and J.J. Magnuson 1977 External mor- Reznick, D. 1982 The impact of predation on life history evo- phological changes associated with sexual maturity in the crayfish lution in Trinidadian guppies: genetic basis of observed life history {Orconectes propinquus). Amer. Mid. Nat. 97: 496-502. patterns. Evolution 36: 1236-1250. Strong, D. R., L. A. Szyska, and D. Simberloff 1979 Tests of Rice, W. 1985 Disruptive selection on habitat preference and evo- community-wide character displacement against null hypotheses. lution of reproductive isolation: an exploratory experiment. Evo- Evolution 33: 897-913. lution 39: 645-656. and D. S. Simberloff 1981 Straining at gnats and swal- Rhoades, R. 1962 Further studies on Ohio crayfishes. Cases of lowing ratios: character displacement. Evolution 35: 810-812. sympatry of stream species in southern Ohio. Ohio J. Sci. 62: , , L.G. Abele, and A. B. Thistle 1984 Eco- 27-33. logical communities: conceptual issues and the evidence. Prince- Rorer, W. E. and G. M. Capelli 1978 Competitive interaction be- ton, NJ: Princeton Univ. Press. tween two Mountain Lake crayfish species, with life history notes. Taper, M. L. and T. J. Case 1985 Quantitative genetic models for Vir. J. Sci. 29: 245-248. the coevolution of character displacement. Ecology 66: 355-371. Rubenstein, D. I. and B. A. Hazlett 1974 Analysis of the ag- Tierney, A.J. and D. W. Dunham 1982 Chemical commu- gressive behavior of the crayfish Orconectes virilis by character analy- nication in the reproductive isolation of the crayfishes Orconectes sis. Behavior 50: 193-216. propinquus and Orconectes virilis (Decapoda, Cambaridae). J. Crust. Salmon, M. 1983 Courtship, mating systems, and sexual selec- Biol. 2: 544-548. tion in Decapods. In: S. Reback and D. W. Dunham (eds.), Studies and 1984 Behavioral mechanisms of repro- in adaptation: the behavior of higher Crustacea. New York: J. ductive isolation in crayfishes of the genus Orconectes. Amer. Midi. Wiley and Sons; pp. 143-171. Nat. Ill: 304-310. Schwartz, F. J., R. Rubelmann, and J. Allison 1963 Ecological Turner, C. 1926 The crayfishes of Ohio. Ohio Biol. Surv. Bull. population expansion of the introduced crayfish, Orconectes virilis. 30: 145-195. OhioJ. Sci. 63: 266-273. Slatkin, M. 1980 Ecological character displacement. Ecology 61: 163-177.